Multifrequency Eddy-Current Method and the Separation of Test Specimen Variables

The multifold accidental degeneracy present in eddy-current signals requires a multifrequency eddy-current examination of specimens to reduce the ambiguity in the determination of the nature of the source of the signals. The multiparameter technique for data analysis first developed by Libby is used for extracting information about multiple variables simultaneously present in the specimen. The effect of interaction between simultaneously present multiple variables on the results of the separation of these variables by this method of linear combination is discussed. It is suggested that the interactions between different regions of a discontinuity result in the lack of a unique relationship between the phase-amplitude characteristics and the depth of discontinuities of unknown geometry and orientation. The difficulty in the estimation of the diameter of the tube support-plate hole in the presence of magnetite (Fe3O4) in the crevice and methods for the determination of the inside dimensions of the tubing in regions with noncircular cross sections are discussed.